1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus capable of adjusting image density of a formed image.
2. Description of the Background Art
Image forming apparatuses typically include an image forming condition control unit to adjust charge bias, development bias, and beam power to a suitable level. The charge bias is applied to a surface of an image bearing member such as a photoconductor drum by a charger. The development bias is an electric potential applied to a development agent supply unit such as a development roller by a development unit. The beam power is a light intensity of light output from an optical writing unit.
The process of adjusting the biases and beam power is generally accomplished by reading a test pattern formed on an image bearing member or the like. With such adjustment process, the image forming operation can be conducted with a given constant image density even if image forming conditions change due to such factors as ambient temperature and humidity during the image forming operation, toner deterioration, photoconductor deterioration, or the like.
The density adjustment process may be conducted as follows. In a case in which the charge bias is fixed at a given value, solid test patterns or solid patterns are formed using a plurality of development biases, change in solid pattern density with respect to the development biases is detected, and a development bias for a suitable density is then set. If the beam power used for forming such solid patterns is such that a surface potential of a latent image of the solid pattern formed on a photoconductor is saturated, the density adjustment can be conducted without problems.
Further, the beam spot diameter in a sub-scanning direction on the photoconductor needs to be set greater than the size of one pixel of a to-be-formed latent image so that a blank area does not occur in the sub-scanning direction of latent image. When the solid pattern is formed using such beam spot diameter, the latent image has a portion in which two pixels overlap, in which the solid pattern saturating the surface potential of the photoconductor can be easily formed using a given beam power.
Then, under the thus-determined development bias, half-tone test patterns or half-tone patterns are formed using a plurality of beam powers, change in half-tone pattern density with respect to the beam power is detected, and a beam power for suitable density of half-tone pattern is then determined. Because the half-tone pattern has fewer overlapping portions on a given latent image, the beam power that can provide a suitable density for half-tone pattern becomes greater than the beam power that forms the solid pattern on the photoconductor that can saturate the surface potential of the photoconductor. If the charge bias is fixed at a suitable level, an image can be formed with a suitable density by conducting the above-described density setting process using the solid pattern and half-tone pattern in the above-described order.
By contrast, in a case in the development bias is fixed at a given value, solid patterns are formed using a plurality of charge biases, change in solid pattern density with respect to the charge bias is detected, and a suitable charge bias is then determined. The subsequent processes are similar to the above-described case in which the charge bias is fixed at a give value.
Further, instead of fixing the charge bias or development bias alone, solid patterns can be formed by setting a plurality of combinations of charge and development biases to select a combination suitable for optimum image density from the plurality of combinations. Further, solid patterns and/or half-tone patterns can be formed using a plurality of combinations of charge bias, development bias, and beam power to select a combination suitable for optimum image density from the plurality of combinations.
It is desirable that image forming apparatuses have a plurality of resolution levels such as, for example, 600 dpi (dots per inch) and 1200 dpi, and such image forming apparatuses having a plurality of resolution levels are already commercially available.
However, conventional image forming apparatuses adapted for a plurality of resolution levels may employ a mechanism or system adapted to a higher resolution level (for example, 1200 dpi when 600 dpi and 1200 dpi are available) among a plurality of resolution levels, by which both the size and the cost of the apparatus increases. Specifically, a larger and more precise optical system is required when it is necessary to set the beam spot diameter on a photoconductor with a higher resolution level compared to an optical system using the beam spot diameter of a lower resolution level. Further, the above described density adjustment conducted for the high resolution level may be also applied to the low resolution level.
Accordingly, to reduce cost, it may be preferable to use a mechanism adapted to a low resolution level, but problems may occur as follows.
For example, if the mechanism is adapted for the low resolution level, the beam spot diameter may become too large when writing one pixel with the high resolution level, by which the image may be blurred or clogged. Such problem can be reduced by conducting a density adjustment.
In general, the light intensity of light beam has its peak at the center of light beam, and the light intensity decreases the farther from the center of light beam. Accordingly, the beam spot diameter is set substantially smaller when conducting the density adjustment to prevent a blurred or clogged image and enable the image to be formed with the high resolution level.
Specifically, when the density adjustment is conducted, a beam power is set smaller or a charge bias is increased to reduce the amount of development agent adhering to the to-be-formed half-tone pattern, by which a blurred or clogged image can be prevented. When the solid pattern is formed, such blurred or clogged image may not become a problem, because the image forming pattern of solid pattern can be formed in the same manner for both the low and high resolution levels.
However, if the beam spot diameter on the photoconductor is adjusted for the low resolution level and the beam power is adjusted to a smaller value, the image forming operation at the high resolution level may require a greater range for light intensity of beam power compared to the image forming operation at the low resolution level, by which a high-power light source may be required. Further, the high-power light source may induce a lower precision when a given light intensity is set. Accordingly, the high-power light source which can set a light intensity with a high precision may be required, but such light source may increase the apparatus cost. Further, if the charge bias is increased, the potential difference between the charge bias and the development bias increases, by which fogging may more likely occur.
Accordingly, if a conventional density adjustment is conducted, the adjustment may not be conducted effectively while the image patterns are formed meaninglessly, and thereby the development agent may be wasted and the adjustment process may become useless.
JP-2009-223215-A may not disclose a method of shifting the resolution level from low to high resolution in an image forming apparatus adapted for using a plurality of resolution levels, by which the above described problems may not be cured.